The present invention generally relates to optical reflectometry, and more particularly, to a system and method for measuring the height differential between two adjacent surfaces.
With the advent of optical reflectometry based measuring devices capable of distances as small as 10 microns (xcexcm), precise and accurate measurements of critically small distances can be made. A nonlimiting example of an optical reflectometry based measuring device is the optical thickness gauge (OTG) once sold by Hewlett-Packard (HP 86125A-K1X). The operation and functionality of such an OTG is disclosed in U.S. Pat. Ser. No. 5,642,196, filed on Jun. 24, 1997, and entitled METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF A FILM USING LOW COHERENCE REFLECTOMETRY, which is entirely incorporated herein by reference. Other exemplary optical reflectometry based measuring devices and their applications, incorporated herein by reference, are disclosed in U.S. Pat. No. 5,473,432, filed on Dec. 5, 1995, and entitled APPARATUS FOR MEASURING THE THICKNESS OF A MOVING FILM UTILIZING AN ADJUSTABLE NUMERICAL APERTURE LENS, U.S. Pat. No. 5,610,716, filed on Mar. 11, 1997, and entitled METHOD AND APPARATUS FOR MEASURING FILM THICKNESS UTILIZING THE SLOPE OF THE PHASE OF THE FOURIER TRANSFORM OF AN AUTOCORRELATOR SIGNAL, U.S. Pat. No. 5,633,712, filed on May 27, 1997, and entitled METHOD AND APPARATUS FOR DETERMINING THE THICKNESS AND INDEX OF REFRACTION OF A FILM USING LOW COHERENCE REFLECTOMETRY AND A REFERENCE SURFACES, U.S. Pat. No. 5,646,734, filed on Jul. 8, 1997, and entitled METHOD AND APPARATUS FOR INDEPENDENTLY MEASURING THE THICKNESS AND INDEX OF REFRACTION OF FILMS USING LOW COHERENCE REFLECTOMETRY, U.S. Pat. No. 5,642,196, filed on Jun. 24, 1997, and entitled METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF A FILM USING LOW COHERENCE REFLECTOMETRY, U.S. Pat. No. 5,731,876, filed on Mar. 24, 1998, and entitled METHOD AND APPARATUS FOR ON-LINE DETERMINATION OF THE THICKNESS OF A MULTILAYER FILM USING A PARTIALLY REFLECTING ROLLER AND LOW COHERENCE REFLECTOMETRY, and U.S. Pat. No. 5,850,287, filed on Dec. 15, 1998, and entitled ROLLER ASSEMBLY HAVING PRE-ALIGNED FOR ON-LINE THICKNESS MEASUREMENTS.
FIG. 1 is a block diagram illustrating a conventional optical thickness gauge (OTG) 100 measuring distances associated with a multi-layer film 102 and in communication with a personal computer (PC) 104. The OTG 100 has at least a low-coherence light source 106, an optical coupler 108, an autocorrelator 110 and a probe head 112. Low-coherence light 114 is generated by the low-coherence light source 106 and injected into waveguide 116. Waveguide 116 may be any suitable device, such as an optical fiber, that is configured to transfer the low-coherence light 114 to the optical coupler 108. The low-coherence light 114 propagates through the optical coupler 108, through the waveguide 118 and into the probe head 112. Light is reflected back into the probe head 112, in a manner described below, through the waveguide 118, through the optical coupler 108, through the waveguide 120. The return light 122 is detected by the autocorrelator 110 so that distance measurements can be determined, as described below, by software (not shown) residing in PC 104.
For convenience of illustration, the waveguide 116 is illustrated as having a separation distance from the low-coherence light source 106. One skilled in the art will appreciate that the waveguide 116 would be typically coupled directly to the low-coherence light source 106 using well known coupling devices (not shown). Likewise, the waveguide 120 is illustrated as having some amount of separation from the autocorrelator 110. Waveguide 120 is typically coupled directly to the autocorrelator 110. For convenience of illustration, waveguide 118 is illustrated as being directly coupled to the optical coupler 108 and probe head 112. Coupling devices used to couple waveguides 116, 118 and 120 to devices are well known in the art and are not described in detail or illustrated herein. Furthermore, for convenience of illustration, waveguides 116, 118 and 120 are illustrated as a rod-like material intended to represent a flexible optical fiber. However, any suitable waveguide device configured to transmit light between the low-coherence light source 106, the optical coupler 108, the autocorrelator 110 and the probe head 112, may be substituted for the waveguides 116, 118 and/or 120.
The optical autocorrelator 110 is configured to receive the return light 122. Detectors (not shown) residing in the autocorrelator 110 provide information such that the autocorrelator 110 generates correlation peaks that are shown on graph 128. Separation between correlation peaks corresponds to distances between any two light reflecting surfaces.
Optical correlator 110 is coupled to PC 104 via connection 124. Information from autocorrelator 110 is received by the PC 104 and processed by software (not shown) into correlation information. PC 104 typically displays, on display screen 126, the correlation results as a graph 128 having correlation peaks, described in greater detail below. That is, distances between correlation peaks correspond to the measurements taken by the OTG 100.
For convenience of illustration, PC 104 is illustrated as a conventional laptop PC. However, any suitable PC or other processing device may be equally employed to provide the necessary processing of information corresponding to the light signals received by the autocorrelator 110, and to prepare a meaningful output format that may be interpreted by a user of the OTG 100 for the determination of distances. Furthermore, display 126 may be any suitable device for indicating distance information resulting from measurements taken by the OTG 100. For example, but not limited to, display 126 may be a conventional, stand-alone cathode ray tube (CRT). Or, a line printer, plotter, or other hard copy device may be configured to accept and indicate correlation information from the autocorrelator 110.
Light (not shown), entering the probe head 112 via waveguide 118, first passes through a reference surface 130. Here, reference surface 130 is illustrated as the bottom surface of a wedge-shaped plate 131. (For convenience of illustration, wedge-shaped plate 131 is shown from an edge-on viewpoint.) Reference surface 130 is configured to allow a portion of the received light to pass through the wedge-shaped plate 131 and onto film 102. A portion of the received light (not shown) entering the wedge-shaped plate 131 is reflected from reference surface 130, back through the probe head 112, through the waveguide 118, through the optical coupler 108 and then through the waveguide 120 to be received by the autocorrelator 110.
FIG. 2 is a simplified graph 200 illustrating the correlation peaks associated with the reflection of light from the reference surface 130 and the surfaces 132, 134, 136 and 138 of film 102 (FIG. 1). For convenience of illustrating the autocorrelation information on graph 200, the vertical axis corresponding to the magnitude of the correlation peaks is not numbered. One skilled in the art will realize that any appropriate vertical axis numbering system corresponding to the amplitude of the correlation peaks could have been employed, and that such a numbering system is not necessary to explain the nature of the correlation peaks. Similarly, the horizontal axis corresponding to distance has not been numbered on graph 200. One skilled in the art will realize that any appropriate axis number system corresponding to distance could have been employed, and that such a numbering system is not necessary to explain the nature of the relationship between the correlation peaks illustrated in graph 200. Thus, one embodiment of the software generating the graph 200 is configured to allow the user of PC 104 (FIG. 1) to alter the horizontal and the vertical axis numbering systems so that the location of the correlation peaks of interest, and their relative separation corresponding to distance, can be meaningfully discerned and determined by the user of the PC 104.
Information received from the autocorrelator 110 is processed by PC 104 (FIG. 1) such that the correlation peak 202 is plotted at the reference point (x=0 on the x-axis) on graph 200. Correlation peak 202 is a large peak, plotted at the zero or reference point on the x-axis of graph 200, that corresponds to the correlation of each the reflected light portions with itself.
Returning to FIG. 1, the portion of light passing through the reference surface 130, referred to as the incident beam 140, passes through air for a suitable distance before striking the first surface 132 of film 102. When the incident beam 140 shines upon surface 132, a portion of the incident beam 140 is reflected from the surface 132, as reflected light beam 142, back up through the probe head 112, through the waveguide 118, through the optical coupler 108, through the waveguide 120, and then is received by the autocorrelator 110. The autocorrelator 110, based upon the time delay between the light reflected from the reference surface 130 and the reflected light beam 142, determines a correlation peak 204 (FIG. 2) as illustrated on graph 200. Typically, the magnitude of the reflected light beam 142 is relatively small such that the correlation peak 204 is significantly less in magnitude than the correlation peak 202, as illustrated in graph 200. The user of PC 104 viewing graph 200 may interpret the relative separation between correlation peaks 202 and 204 as corresponding to a distance 144 between the reference surface 130 and the surface 132 of film 102.
For convenience of illustration, the incident beam 140 and the reflected light beams 142, 154, 158 and 162 are shown at slight angles. However, one skilled in the art will appreciate that the incident beam 140 and light beams 142, 154, 158 and 162 are all orthogonal to the reference surface 130 and the surfaces 132, 134, 136 and 138. Furthermore, for convenience of illustration, because the distance 144 is typically much greater than the distances of interest associated with film 102, only a portion of the distance between the correlation peaks 202 and 204 is illustrated. Thus, a portion of the horizontal axis and a portion of the distance between correlation peaks 202 and 204 is omitted from graph 200, as indicated by the break line 206.
One skilled in the art will appreciate that the separation between the correlation peaks 202 and 204 is a function of a variety of well known physical factors. Light travels at a finite speed. The speed of the light is affected by the medium through which the light is traveling. Thus, one skilled in the art will readily appreciate that two significant factors in determining the time delay of the various portions of light detected by the autocorrelator 110 are the total distance traveled by the light, and the properties of the various medium through which the light travels. For example, the reflected light beam 142 travels from the reference surface 130 to surface 132, and then returns back to the reference surface 130. Therefore, because reflected light beam 142 travels farther than the light reflecting from the reference surface 130, and because the light beam 142 travels through air, the light beam 142 requires more time to reach the autocorrelator 110 than the time required by the light reflecting from the reference surface 130. The physical properties associated with the mediums through which the light travels is defined by the well known refractive index (n) of the material. Thus, software analyzing the relative separation between correlation peak 202 and correlation peak 204 accurately calculates the distance 144 and provides that information to the user of PC 104. This information may be communicated by appropriately labeling the horizontal axis of FIG. 2, and/or providing a numerical figure to the user. Such a process of determining distances with an OTG 100 (FIG. 1) is well known in the art and is not described in further detail herein.
FIG. 1 illustrates the OTG 100 measuring distances associated with film 102. For convenience of illustration, film 102 has three layers; a top layer 146, a middle layer 148 and a bottom layer 150. The layers 146, 148 and 150 are made from different materials bonded together to create a single layer of film 102. Typically, film 102 is a long, continuous roll or sheet of flexible material. However, for convenience, only a portion of the roll or sheet of film 102 is shown in FIG. 1, as illustrated by the cut-away lines 152. Furthermore, the layers 146, 148 and 150 must be sufficiently transparent such that incidence beam 140 travels through, and light is reflected back through the layers 146, 148 and 150.
Each layer 146, 148 and 150 have different refractive indices (n). Surface 132 corresponds to the transition between air and the film 102, and thus corresponds to a change in the refractive index of air to the refractive index of the top layer 146. Similarly, surface 134 corresponds to the transition between the material of top layer 146 and the material of middle layer 148. Surface 136 corresponds to the transition between the middle layer 148 and the bottom layer 150. Surface 138 corresponds to the bottom surface of film 102, and also corresponds to a transition between the bottom layer 150 and the material that the film 102 is residing in, such as air. Each of these surfaces may also be characterized by a change in refractive indices.
When incidence beam 140 is incident on surface 134, a portion of the incidence beam 140 passes through the surface and a portion of the incidence beam 140 is reflected back up to the probe head 112 because of the difference in the refractive indices n of the layers 146 and 148. The amount of reflected light corresponds, in part, to the degree of difference between the refractive indices n. Thus, when incidence beam 140 passes through top layer 146 into the middle layer 148, reflected light beam 154 is reflected from the surface 134 back up through the top layer 146 and into probe head 112. The reflected light beam 154 is eventually detected by the autocorrelator 110 in the manner described above. Because of the time delay between the reflected light beam 154 from the surface 134 with respect to the light reflected from reference surface 130, a correlation peak 208 (FIG. 2) will be determined. Furthermore, since the time delay between the reflective light beam 154 from the surface 134, with respect to the reflective light being 142 from the surface 132, is equal to the time required for the light to travel through the layer 146 only, the separation between the correlation peak 204 and correlation peak 208 (FIG. 2) is proportional to the distance 156 and the index of refraction of the layer 146.
Similarly, a portion of incidence beam 140 incident on the surface 136, corresponding to the material transition between the middle layer 148 and the bottom layer 150, is reflected back up to the probe head 112 as reflected light beam 158. Because of the time delay associated with the reflected light beam 158 with respect to the light reflected from reference surface 130, a correlation peak 210 (FIG. 2) is determined. Furthermore, since the time delay between the reflective light being 158 from the surface 136, with respect to the reflective light being 154 from the surface 134, is equal to the time required for light to travel through the layer 148 only, the separation between the correlation peak 208 and the correlation peak 210 is proportional to the distance 160 and the index of refraction of the layer 148.
Likewise, a portion of incidence beam 140 will be reflected from surface 138 back up to the probe head 112 as reflected light beam 162. Because of the time delay associated with the reflected light beam 162 with respect to the light reflected from reference surface 130, a correlation peak 212 (FIG. 2) is determined. Furthermore, since the time delay between the reflective light beam 162 from the surface 138, with respect to the reflective light being 158 from the surface 136, is equal to the time required for light to travel through layer 150 only, the separation between the correlation peak 210 and the correlation peak 212 is proportional to the distance 164 and the index of refraction of the layer 150. In some applications, the bottom surface 138 of the film 102 is coated with a highly reflective surface such that a large portion of the incidence beam 140, or all of the remaining incidence beam 140, is reflected up to the probe head 112 as reflected light beam 162. Thus, the correlation peak 212 is illustrated as having a relatively greater magnitude than the correlation peaks 204, 208 and 210 (FIG. 2).
For convenience of illustrating graph 200 (FIG. 2), not all correlation peaks are illustrated. Autocorrelator 110 (FIG. 1) generates a correlation peak for all pairs of reflections from any two surfaces. For example, the autocorrelator 110 determines a correlation peak associated with the reflected light beam 154 and the reflected light beam 158 (FIG. 1). Another example includes a correlation peak associated with the reflected light beam 154 and the reflected light beam 162 (FIG. 1). One skilled in the art will appreciate that many correlation peaks (not shown for convenience of illustration) will be displayed on graph 200, and that one skilled in the art will employ experience in using the OTG 100 (FIG. 1) to determine which correlation peaks are relevant to the particular measurements of interest. Thus, for convenience of illustration, the correlation peaks illustrated on graph 200 are limited to peaks that are convenient in explaining the operation and functionality of the OTG 100.
Summarizing, the OTG 100 shines a low-coherence incidence beam 140 onto the film 102 such that portions of the incidence beam 140 are reflected back to the OTG (reflected light beam 142, 154, 158 and 162) and detected by the autocorrelator 110. Software analyzes the time delays associated with the reflected light beam 142, 154, 158 and 162, with respect to the light reflected from reference surface 130, to determine the distances 144, 156, 160 and 164, respectively. The ability to resolve the minimum peak separation is determined by the coherence-length of the light source. Thus, a lower coherence length light source gives a higher resolution. One commercially available OTG is capable of discerning distances as small as 10 xcexcm.
However, such an OTG 100 is not capable of measuring with any degree of reliability and accuracy other types of material configurations. One such material configuration is illustrated in FIG. 3. FIG. 3 is a block diagram illustrating the OTG 100 attempting to measure distances associated with the top surface 302 of material 304 and the top surface 306 of material 308. For convenience of illustration, only the ends of materials 304 and 308 are shown, as indicated by the cut-away lines 310 and 312, respectively.
Materials 304 and 308 are aligned adjacent to each other. Here, it is desirable to measure the relative vertical positioning of the top surfaces 302 and 306 with respect to each other. Proper vertical positioning of the materials 304 and 308 with respect to each other may be of interest, particularly where the positioning of the materials 304 and 308 must be within a predefined tolerance with respect to each other to ensure proper functionality of materials 304 and 308 in a system (not shown). For convenience of illustration, an end surface 314 of material 304 and an end surface 316 of material 308 are illustrated as being in close proximity to each other, but not touching. Here, it may be desirable to precisely measure the relative positioning of the materials 304 and 308 with respect to each other by measuring the position of surfaces 302 and 306. If the alignment of material 304 and 308 is within a predefined tolerance, it may be desirable to then join end surfaces 314 and 316, by welding or by application of a suitable adhesive, such that the materials 304 and 308 are joined together in a precise manner.
OTG 100, as will be appreciated by one skilled in the art, is not well suited for making the precise measurements necessary to accurately determine the relative positioning of surface 302 with respect to surface 306. That is, reflected light from the surfaces 302 and 306 may not provide for the determination of suitable correlation peaks such that the relative positioning of surfaces 302 and 306 can be determined.
Here, light 318 from the probe head 112 is shined onto materials 304 and 308. Light 318 is intended to correspond to incidence beam 140 (FIG. 1). However, for convenience of illustration, light 318 is illustrated as a beam of light (by the plurality of arrows) emanating from probe head 112. The beam of light 314 has a predefined cross-sectional area, known as the spot size. Spot size is determined by the optics (not shown) designed into the probe head 112. In some applications, a large spot size is undesirable in that a more precise angular positioning of the probe head 112 over the material(s) being measured is required, thus making the OTG 100 more difficult to properly align over the material.
Light 318 results in light reflected from the materials 304 and 308 back up into the probe head 112 in a manner described above. Reflected light 322 is reflected from surface 302 of material 304 up to probe head 112. Similarly, reflected light 324 is reflected from the top surface 306 of material 308 up to probe head 112. Reflected light 322 and 324 are desirable light reflections in that correlation of the reflected light 322 and 324 with respect to the light reflected from reference surface 130 provides the time delay information to determine the distances 326 and 328, respectively. For the reliable and accurate determination of distances 326 and 328, the reflected light 322 and 324, respectively, must have a sufficient magnitude to generate meaningful correlation peaks that can be discerned from the correlation peaks caused by other reflected light.
Material 308 is illustrated as having a rounded corner surface 330 which joins top surface 306 with end surface 316. Comer surface 330 may cause reflected light 334 to be reflected away from the probe head 112 such that the total amount of reflected light returning to probe head 112 is decreased. In practical applications where two materials are to be joined, surface imperfections such as chips, scratches or other deformations are frequently encountered. Such imperfections may have as similar undesirable effect on the reflection of light up to the probe head 112 as described above for the corner surface 330.
Furthermore, any spatial separation between the end surface 314 and the end surface 316 will result in portions of light 318 to pass through the spatial separation, thereby decreasing light reflection back up to probe head 112. This light 336 passing through the spatial separation effectively reduces the total amount of reflected light in an undesirable manner. Furthermore, the spot size 320 may be so narrow that a majority of the light 318 passes through the spatial separation between end surfaces 314 and 316 such that insufficient reflected light is available for determining correlation peaks associated with the surfaces 302 and 306. In such a situation, the spot size 320 could be increased, through suitable optics design, to increase the coverage area of the beam of light 318 onto both surfaces 302 and 306. However, as one skilled in the art will appreciate, an increased spot size 320 necessarily increases the requirement for angular alignment between the axis of the measurement beam and the measured surfaces. Such a requirement of a more precise angular alignment is undesirable since reliable and accurate operation the OTG 100 becomes more difficult.
Thus, a heretofore unaddressed need exists in the industry for providing a system and method of accurately and reliably measuring surface height differentials with optical reflectometry based measuring devices, such as a conventional OTG.
The present invention overcomes the inadequacies and deficiencies of the prior art as discussed hereinabove. The present invention, a split-beam optical thickness gauge (OTG), provides a system and method for measuring the difference in heights of two adjacent surfaces. The split-beam OTG has at least a low-coherence light source, an optical coupler, an autocorrelator and a split-beam probe head. Low-coherence light is generated by the low-coherence light source and injected into a waveguide for transmission to the optical coupler. The split-beam probe head receives the low-coherence light, from the optical coupler via another waveguide, and splits the incoming low-coherence light into a primary beam and walk-off beam.
Two materials are positioned adjacent to each other. The primary beam shines upon the top surface of the first material. Light from the primary beam is reflected back up into the split-beam probe head. The walk-off beam shines upon the top surface of the second material. Light from the walk-off beam is reflected back up into the split-beam probe head. Spatial separation between the primary beam and the walk-off beam ensures that each beam shines substantially on only one of the surfaces. The reflected light returns to the autocorrelator and is detected so that distance measurements can be determined based upon the time delay of the walk-off beam with respect to the primary beam.
In one embodiment, the autocorrelator determines correlation peaks that are plotted on a graph. The difference between the position of the correlation peaks corresponds to the height difference between the two surfaces. The graph is displayed on display residing on a personal computer, such as a laptop.
The present invention can also be viewed as providing a method for measuring heights of two adjacent surfaces. The method includes the steps of splitting a beam of energy into a primary beam and a walk-off beam; transmitting the primary beam onto a first surface and the walk-off beam onto a second surface; detecting reflections of the primary beam from the first surface and the walk-off beam from the second surface; and determining a distance between the first surface and the second surface based upon an optical path length difference between the detected reflections of the primary beam and the walk-off beam.
Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following detailed description, when read in conjunction with the accompanying drawings. It is intended that all such features and advantages be included herein within the scope of the present invention and protected by the claims.